Hot Topics: Insulating Basement Walls

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Original Post: Insulating Basement Walls

CarolusP Member

I'm beginning a project to finish my basement. The house has a block foundation. I'm trying to decide on the best way to insulate the outside walls. From my reading, it sounds like batt insulation has fallen out of favor for basements, so I'm probably looking at closed cell rigid insulation or closed cell spray foam (or some combination of the two). I live in Minnesota, so I need R-15 on the outside walls.

The benefit of rigid insulation is that it’s cost-effective, but the downside is that I lose three inches (in addition to the four inches I'll lose to framing) of the room in order to meet the R-15 requirement. The benefit of spray foam is that I don't lose the three inches of space, but it will probably cost three to four times as much as the rigid foam insulation.

I'm self-learning all of this stuff for the first time, so for those of you who are more experienced, what would you do? Could I do some combination of the two? (One inch of rigid, and then spray foam between the studs?) Are there other options I should consider?

Hal_S Member

Well, the trend that I've seen over the last few years is "a room within a room" design. Instead of studding out the block basement walls and putting in Sheetrock and insulation, leave a 2' to 3' gap between the block wall and new wall. This allows you enough space to access the sewer lines and pipes and doesn't reduce the finished square footage THAT much.

Effectively, you get additional insulation from the dead air between the cold block wall and the new finished wall. Instead of conduction (being in contact with a cold wall), you're only dealing with convection (being in contact with air that is in contact with a cold wall).

Marq1 Member

“It sounds like batt insulation has fallen out of favor for basements.”

No, only when people do not follow common sense installation and seal walls with moisture barrier materials, which allow water to collect and can cause mold. Build the walls correctly. Allowing ventilation and batt insulation is a very cost-effective way to insulate the basement, which is something you can not say about foam!

CarolusP Member

I certainly like the price point of batt. If I went that route, what should/shouldn't I do regarding waterproofing?

Marq1 Member

I'm assuming you are referring to the vapor movement and not a water issue. Regardless of the method used, you have to have a dry basement. Assuming that is correct, all you need to do it build interior walls away from the basement wall, not install any type of plastic, and keep some gaps above the wall to allow air movement, including moisture. If you trap the vapor, it turns into water and then you have a problem!

CarolusP Member

Unfortunately, I don't think that's an option for me. Our local code requires waterproofing/moisture barrier between the frames and the wall. Wouldn't that require something else in between other than just space?

Stickshift Group Moderator

Current convention is rigid foam on the wall and then batts or mineral wool in the framing with no vapor barrier on below-ground walls. The foam acts as a vapor retarder and thermal break to prevent condensation. That said, you're stuck with local codes.

Mossman Member

You want to keep warm moist air from condensing on the cold foundation wall. Rigid foam against the concrete wall (continuous with no breaks, tongue and groove, seams taped) followed by your stud wall with unfaced batts between will accomplish this.

CarolusP Member

So ideally, there's no vapor barrier at any point between the rigid foam and the Sheetrock?

Mossman Member

The rigid foam acts as the vapor barrier, keeping moist indoor air away from the cold foundation wall. In other words, you are drying to the inside.

I believe another (cheaper) way to do it would be to leave an air gap between the stud wall and foundation wall (like others have mentioned) and install paper-faced batts with the vapor barrier towards the foundation wall. I believe the issue with this approach is you will never get a perfectly sealed vapor barrier and warm moist air will find its way between the walls and condensate on the foundation wall, causing mold, etc.

Mossman Member

“You will never get a perfectly sealed vapor barrier and warm moist air will find its way between the walls and condensate on the foundation wall, causing mold, etc.”

The intent is that you don’t close off an area behind the wall by leaving areas open at the top near the rim joist so that you have air flow.

CarolusP Member

Thank you for all of the replies. I have another question (I'm not going to promise it's the last one). I have a block foundation, and the tops of the walls are not capped. I'm planning to put some rigid foam up in the rim joists for insulation, but should I also be laying some foam horizontally over top of the block openings to cover those up?

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